Why sandcastles are so easy to build

EVERY child knows that you don’t have to follow an exact recipe to build a sandcastle. All you need is sand plus a splash of water – and now we understand why.

Mario Scheel at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, and colleagues studied how a liquid squeezes between grains. Using a series of X-ray images to build up a three-dimensional picture of the sand pile, they found that with the driest sand-and-water recipe, the grains were linked by liquid bridges shaped like a double-ended trumpet (Nature Materials, DOI&colon; 10.1038/nmat2117). When enough of these bridges form, the mixture is able to hold its shape.

From then on, unless the mixture becomes saturated, adding more liquid doesn’t make much difference. “The liquid goes into the crevices and fuses the bridges together,” explains Martin Brinkmann, one of Scheel’s collaborators. As long as the bridges retain something of their original shape their mechanical properties remain the same, he says.

The result could be applied to many different liquids and granular materials, and help us understand other mixing processes. It might even help to determine the critical point at which a combination of mud and water can become a landslide.

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